Method and base station for automatic carrier selection

ABSTRACT

A femtocell basestation selects its carrier frequency by detecting a carrier used by at least one other basestation in its network, and selecting its own carrier frequency such that it partially overlaps with the detected carrier. The downlink power of the femtocell basestation is set based on the degree of overlap between the selected carrier and the detected carrier, so that a user equipment having a connection to the other basestation is forced to search for the selected carrier.

This application is a continuation of U.S. patent application Ser. No.13/578,558, filed on Oct. 26, 2012, which is a 371(c) application ofPCT/GB2011/050259 filed Feb. 11, 2011, entitled “METHOD AND BASE STATIONFOR AUTOMATIC CARRIER SELECTION”, which claims priority to GB 1002397.6,filed Feb. 12, 2010, and entitled “CARRIER SELECTION”, each of whichprior application is hereby incorporated by reference and for allpurposes.

This invention relates to a basestation, for use in a cellular mobilecommunications network, and in particular to the way in which such abasestation can select its operating carrier frequency.

Femtocell basestations are known, in the form of access points thatprovide mobile coverage in areas where coverage is problematic, forexample indoors. Such devices are typically intended to runautonomously, and thus have many self-configuration capabilities.

Femtocell basestations are expected to complement the existingmacrolayer coverage, so that the same devices should be able to attachto and use macrolayer basestations and femtocell basestations, dependingon their location. In order for mobile users to be able to roam betweenthe macro layer and the femto layer, there has to be a mechanism bywhich a handset that is attached to a macrolayer basestation can searchfor femtocell coverage. Then, when femtocell coverage is detected, thehandset can attach to the femto layer and get the benefits of theimproved coverage and additional capacity provided by the femtocellbasestations.

When a femtocell basestation has the same radio channel frequency as themacro layer in its immediate vicinity, mobility between the macro layerand the femtocell is easily achieved. Because the two layers share thesame frequency, a mobile camped on (or connected to) the macro networkwill experience a degradation of macro quality when it is in thevicinity of a co-channel femtocell. This degradation will trigger themobile to search for better cells in the area as defined in theNeighbour List. Thus, the co-channel deployment will force the mobile tosearch and find femtocells without having to change mobility parameterson the macro network. However, the co-channel deployment createsinterference between the two layers which can be harmful, particularlyfor a mobile user that is unauthorised to use a femtocell that iscausing interference in the macro layer or no service at all because offrom a femtocell nearby. Unauthorised handsets can also experienceadditional disadvantages, particularly if they are R6 compliant. Whensuch a device is rejected on the femto layer, the channel is barred forup to 300 seconds, which means that the handset will not be able to usethe macro network for that duration, even if the macro quality isadequate.

When the femtocell basestations in a network are allocated a dedicatedfrequency, mobility between the macro layer and the femto layer isachieved in a different way. In the Idle mode, inter-frequency cellreselection parameters on the macro network instruct the handsets tosearch for specific scrambling codes used by the femtocell basestations.In the connected mode, the macro network instructs the handsets to gointo “compressed mode”, in order to search for and measure any cellsthat happen to be using the femtocell frequency. In compressed mode, thehandset periodically stops transmission, and momentarily tunes itself toanother frequency to measure the quality of that frequency. The use of adedicated frequency for the femtocell basestations eliminates theinterference problem. However, the changes to mobility parameters on themacro network will cause all handsets camped on a macro layerbasestation to try to detect the femtocell layer. Using compressed modeacross the whole network is usually unacceptable as it leads to capacityand quality problems.

In some cases, the macro layer network has a number of carriers(channels), and therefore, when femtocell basestations are deployed onthese carriers, all of the mobility issues described above will appear.

According to the present invention, there is provided a method ofoperation of a basestation in a mobile communications network, whereinthe mobile communications network comprises a plurality of otherbasestations, each operating in a respective carrier frequency band, themethod comprising:

-   -   detecting a carrier in use by at least one of the other        basestations, wherein the detected carrier has an effective        bandwidth, and    -   selecting a carrier having said effective bandwidth for use in        the basestation, based on the detected carrier in use by the        other basestation, such that the selected carrier partially        overlaps with the detected carrier in use by the other        basestation.

According to a second aspect of the present invention, there is provideda basestation adapted to operate in accordance with the method of thefirst aspect.

For a better understanding of the present invention, and to show how itmay be put into effect, reference will now be made, by way of example,to the accompanying drawings, in which:

FIG. 1 illustrates a part of a cellular communications network operatingin accordance with an aspect of the present invention.

FIG. 2 is a block schematic diagram, illustrating a femtocellbasestation in accordance with an aspect of the present invention.

FIG. 3 is a flow chart, illustrating a method in accordance with anaspect of the present invention.

FIG. 4 illustrates a frequency allocation in an aspect of the invention.

FIG. 5 illustrates a frequency allocation in another aspect of theinvention.

FIG. 6 illustrates a frequency allocation in yet another aspect of theinvention.

FIG. 7 illustrates a frequency allocation in yet another aspect of theinvention.

FIG. 8 illustrates a frequency allocation in yet another aspect of theinvention.

FIG. 1 shows a part of a cellular communications network. At this levelof generality, the network is conventional, and will be described onlyso far as is necessary for an understanding of the present invention. Inthe illustrated cellular network, a femtocell basestation, or femtocellaccess point (FAP) 10 has been deployed. The FAP 10 is in the vicinityof three macro layer cellular basestations 12, 14, 16 within the samecellular network. It will be appreciated that a practical network willinclude many more basestations, but the present invention can bedescribed sufficiently without illustrating additional basestations.

FIG. 2 illustrates in more detail the FAP 10. Again, at this level ofgenerality, the FAP 10 is known, and will be described only so far as isnecessary for an understanding of the present invention. The FAP 10operates under the control of a processor 20, which monitors, andcontrols the operation of, the other components of the FAP 10. Thecommunication with the core network of the cellular network operator istypically over the internet, by means of an interface 22. Signals to andfrom the interface 22 for communication over the wireless interface tomobile devices or other user equipment are passed to a modem 24, whichputs the signals into the appropriate format, based on the relevantcellular standard. The invention will be described further withreference to a FAP 10 operating in accordance with the UMTS cellularstandard, but it will be apparent that the invention can be applied toany other appropriate standard.

Signals received by the FAP 10 over the wireless interface are passed toconventional receive (Rx) circuitry 26, operating at a frequency that isderived from a signal received from an oscillator 28. Signals fortransmission over the wireless interface are passed to conventionaltransmit (Tx) circuitry 30, operating at a frequency that is derivedfrom a signal received from an oscillator 32. The oscillators 28, 32 areshown here as separate, but there may instead be a single oscillator,with the Rx circuitry 26 and the Tx circuitry 30 deriving the relevantreceive and transmit frequencies from that single oscillator. Connectedto the Rx circuitry 26 and the Tx circuitry 30 is an antenna 34.

FIG. 3 is a flow chart, illustrating a process performed by the FAP 10,under the control of the processor 20, during the initialisation of thedevice. The same process can be carried out at regular intervals whilethe FAP 10 is in operation, to take account of changes in the signalstransmitted by macro layer basestations in the vicinity.

In step 50, the FAP 10 attempts to detect downlink transmissions frommacrocell basestations in the cellular network. In step 52, the FAPselects a channel. The selection is based on information that isobtained in step 50.

Thus, the macrolayer basestations are typically all operating oncarriers that are determined by the cellular network operator as part ofits network planning. For example, two carriers might be allocated toall of the macrolayer basestations in the network. In that case, the FAP10 might be able to detect one or more macrolayer basestation operatingon a first carrier, or might be able to detect one or more macrolayerbasestation operating on a second carrier, or might be able to detectmacrolayer basestations operating on the first carrier and the secondcarrier. In one embodiment, the FAP 10 is able to download informationfrom a management system provided by the cellular network operator,informing it what action to take in any of these situations.

For example, the FAP 10 might have downloaded information, instructingit that: if it is only able to detect one or more macrolayer basestationtransmitting on a first carrier, it should transmit on a third carrier;if it is only able to detect one or more macrolayer basestationtransmitting on a second carrier, it should transmit on a fourthcarrier; or, if it is able to detect macrolayer basestationstransmitting on the first carrier and the second carrier, it shouldtransmit on a fifth carrier. In each case, there is a partial overlapbetween the carrier on which the FAP 10 transmits and the carrier orcarriers on which the detected macrolayer basestation transmits.

In another embodiment, the FAP might select a carrier based on adifferent algorithm, depending on the number of macrolayer basestationsthat it is able to detect, the carriers that they are transmitting on,and the strengths of the detected macrolayer carriers.

For example, the FAP might determine a channel offset, and then select acarrier that is separated from the carriers of the detected macrolayerbasestations by that carrier offset. Again, the channel offset is suchthat there is a partial overlap between the carrier on which the FAP 10transmits and the carrier or carriers on which the detected macrolayerbasestation transmits.

Having selected the carrier that it will use for its downlinktransmissions, the FAP 10 is readily able to determine the carrier to beused for uplink transmissions, because there is a fixed relationshipbetween the downlink and uplink carriers for all devices in the UMTSsystem. In the UMTS system, the channel raster is 200 kHz, that is, theavailable frequency channels are spaced apart by 200 kHz, and eachchannel is given a number, the UTRA Absolute Radio Frequency ChannelNumber (UARFCN), where the frequency of a channel is its UARFCNmultiplied by 200 kHz.

In UMTS, transmitters and receivers operate using carriers, which arenominally 5 MHz wide. That is, any transmitter and receiver will beoperating on a carrier, which is identified by the UARFCN thatcorresponds to the centre frequency of the carrier, but the limits onfrequency selectivity in the various filters, etc mean that the carrierwill contain components over a frequency range of 5 MHz centred on thatcentre frequency.

Thus, FIG. 4 shows a situation where a basestation is transmitting onchannel number 10590. That is, it is generating signals across abandwidth of approximately 5 MHz centred on 2118 MHz, which is theproduct of the channel number, 10590, and 200 kHz.

FIG. 5 shows a typical situation where there are two basestationsoperating on two different carriers, with a first basestation istransmitting on channel number 10590 and a second basestationtransmitting on channel number 10615. That is, the first basestation isgenerating signals across a bandwidth of approximately 5 MHz centred on2118 MHz, while the second basestation is generating signals across abandwidth of approximately 5 MHz centred on 2123 MHz, which is theproduct of the channel number, 10590, and 200 kHz.

FIG. 6 shows the result of step 52, in the case where the FAP 10 is ableto detect only one or more macrolayer basestation transmitting on thecarrier 80 centred on channel number 10590. Based on the informationprovided by the network operator's management system, the FAP 10 selectsthe carrier 82, centred on channel number 10608 for its owntransmissions. There is thus a channel offset C*, where C*=18 in thisillustrated example, between the centre frequencies of the two carriers.

The difference between the centre frequencies of the two carriers is thechannel offset multiplied by 200 kHz. Thus, where the channel offset isselected to be less than 25, the difference between the centrefrequencies is less than 5 MHz. As each carrier has an effectivebandwidth of 5 MHz, there is a partial overlap between the carriers 80,82.

FIG. 7 shows the area 84 of overlap between the carriers 80, 82.

The difference between the centre frequencies of the two carriers is thechannel offset multiplied by 200 KHz. Thus, where the channel offset isselected to be less than 25, the difference between the centrefrequencies is less than 5 MHz. As each carrier has an effectivebandwidth of 5 MHz, there is a partial overlap between the carriers 80,82.

FIG. 8 shows the result of step 52, in the case where the FAP 10 is ableto detect one or more macrolayer basestation transmitting on the carrier80 centred on channel number 10590, and one or more macrolayerbasestation transmitting on the carrier 86 centred on channel number10615. It will be seen that the difference between the centrefrequencies of the carriers 80, 86 is 5 MHz, which is a common situationwhen a network operator provides two carriers for its macrolayerbasestations. In this case, the FAP 10 selects a carrier that partiallyoverlaps with both of the carriers 80, 86.

Specifically, in this example, based on the information provided by thenetwork operator's management system, the FAP 10 selects the carrier 88,centred on channel number 10603 for its own transmissions. There is thusa channel offset C*¹, where C*¹=13 in this illustrated example, betweenthe centre frequencies of the carriers 80 and 88, and a channel offsetC*², where C*²=12 in this illustrated example, between the centrefrequencies of the carriers 86 and 88.

Again, these two channel offsets are selected to be less than 25, and sothe differences between the pairs of centre frequencies are less than 5MHz. As each carrier has an effective bandwidth of 5 MHz, there is apartial overlap between the carriers 80 and 88, and a partial overlapbetween the carriers 86 and 88.

Thus, the selected carrier straddles the guard band that separates thecarriers 80, 86.

The selection of a carrier that partially overlaps with a macrolayercarrier combines the benefits of operating on the same carrier as themacrolayer basestations in the vicinity and of operating on a separatecarrier from the macrolayer basestations, while minimising thedisadvantages.

Thus, while there is some interference with the macrolayer carrier, theinterference is limited. In fact, the degree of interference is afunction of the degree of overlap between the two carriers. A largerchannel offset reduces the amount of interference. The channel offset istypically at least 4, i.e. the centre frequency spacing is at least 800KHz. At the same time, the channel offset is typically less than 21,i.e. the centre frequency spacing is typically less than 4.4 MHz. Thismeans that the amount of interference does not adversely affect signalreception for most users, but does create a sufficient level ofinterference that degrades the macro carrier in a small region aroundthe femtocell, and thus forces the user to search for and reselect tothe femto layer. Because the interference is substantially less than inthe case of the shared carrier, this small region around the femtocellcan in practice be made small enough that a user moving into thecoverage area of the femtocell will only start searching for thefemtocell when it is close to the femtocell.

By configuring the inter-frequency cell reselection parameters on themacro network, only handsets that experience a certain level of qualitydegradation will then search for the femto carrier. This substantiallydecreases the proportion of handsets that are searching for the femtolayer. It means also that the proportion of connected handsets (in call)that need to be put in compressed mode to search for the femto carrieris substantially reduced. This increases average battery life for users,and minimises the reduction in the macrolayer capacity caused by the useof compressed mode.

Also, selecting a partially overlapping carrier for use by a FAP meansthat unauthorised macrolayer users who attempt to access the FAP and getrejected are more likely to find a macro layer basestation withacceptable quality to return to. In addition, the channel blockingprocedure, whereby a user is prevented from using a carrier on which ithas been rejected, has no effect on the availability of the macro layer,because the blocked carrier is different from the one used by the macronetwork.

Having selected a carrier, the process shown in FIG. 3 passes to step54, in which it sets the transmit power levels for the FAP 10.Specifically, based on the measured Received Signal Code Power (RSCP)levels of the detected macro layer carriers, the FAP 10 chooses adownlink power setting that will degrade the macro signal quality(Ec/Io) to a pre-determined level at a certain pre-determined distance(or path loss) and for the chosen channel offset. The FAP has to createan adequate level of interference that causes the handset to search forthe FAP when it is at a certain distance (e.g. 2-3 meters) from the FAPwhile, at the same time, ensuring that the interference generated by theFAP should not cause a pronounced quality degradation to the macrolayer.

For example, the transmission power level Tx power may be determinedaccording to the equation:Tx power=measured RSCP+PL−Ec/Io+LF+CF,where:

measured RSCP is the RSCP level of the strongest detected signal fromany macrolayer basestation;

PL is the desired value for the path loss at the point at which thetransition between the macro layer and the femto layer takes place, andmay for example be set to 50 dB, so that the transition only occursclose to the FAP 10;

Ec/Io is the macro layer Ec/Io quality at which the UE is triggered tosearch for the FAP UARFCN, and the value of this can be read by the FAP10 off the macro layer broadcast channel;

LF is a loading factor that takes into account the loading differencebetween the two layers, as well as the proportion of FAP power allocatedto the Common Pilot Channel (CPICH); and

CF is a correction factor that is a function of the channel offset C*between the FAP 10 and the adjacent macro layer basestation.

In principle, a larger correction factor should be applied for largervalues of the channel offset. However, the power setting procedure inthe FAP can also take into consideration the level of coverage providedby the FAP to FAP UEs, the interference caused by the FAP to any otheradjacent/overlapping carriers, and the maximum power capabilities of theFAP.

For purposes of example and illustration only, the followingrelationship between the channel offset C* and the correction factor CFis possible:

Channel Correction offset Factor (dB) <8 0 8 1.5 9 2.5 10 3.0 11 7 1210 >12 13

Having set the transmit power level, the procedure of FIG. 3 passes tostep 56, in which the FAP 10 also sets the maximum power allowed byhandsets in the uplink. The FAP has to control the interferencegenerated by the FAP UEs into the partially overlapping macro layercarrier.

For example, in one embodiment, the FAP calculates the path loss to thenearest macro layer base station. This is based on its measurement ofthe macro RSCP and the Transmit Power of the macro layer pilot signal,which is known from transmissions over the broadcast channels. The FAPthen calculates the maximum co-channel transmit power that will create apre-configured level of uplink noise rise at the nearest macro layerbase station. The FAP then uses another mapping table, similar to theone above, to correct the difference in power for an offset channel. TheFAP then broadcasts the calculated value for the maximum uplink powerover its broadcast channel.

Where the FAP carrier partially overlaps with two carriers in the macrolayer, it can adjust the downlink and uplink powers in steps 54 and 56based on measurements of the stronger overlapping carrier, or based on ameasurement of one particular overlapping carrier notified by themanagement system, or based on an averaging function that usesmeasurements from the two overlapping carriers.

The procedure of FIG. 3 passes to step 58, in which the FAP 10configures the various inter-frequency cell reselection parameterstowards the offset macro carrier(s). As an example of this, the FAPconfigures the compressed mode thresholds to cause Femto handsets tosearch for and handout to macro sites on offset carriers. The FAP 10 hasto configure the inter-frequency cell reselection parameters in order toenable seamless transition from the femto layer to the macro layer atthe right boundary. The configuration is specific to that particular FAPbecause it is dependent on the strength of the macro signal at thatlocation, and is also dependent on the desired extent of coverage of theFAP.

Having set the various parameters as described, the process passes tostep 60, and the FAP 10 is then able to begin operation.

Thus, the femto layer configuration uses a UMTS channel which is spacedless than 5 MHz to the adjacent macro channel. The femto layerconfiguration uses a carrier that overlaps one or two macro carriers.This generates a pre-determined and controlled level of interference.The self-configuring FAP adjusts its power settings based on ameasurement on an adjacent and overlapping carrier. The self-configuringFAP controls the power of connected handsets based on measurements on anadjacent and overlapping carrier. The self-configuring FAP adjustsreselection parameters based on measurements on an adjacent andoverlapping carrier. The FAP instructs UEs in connected mode to measureone or more UARFCNs that belong to overlapping carriers belonging to themacro network using compressed mode.

This therefore provides a mechanism to deploy femtocells without waitingfor a newer generation of handsets or equipment.

The invention claimed is:
 1. A method of operation of a basestation in acellular communications network, the method comprising, in thebasestation: detecting signals transmitted by other basestations of thecellular communications network; identifying carrier frequencies onwhich the detected signals are being transmitted by the otherbasestations; selecting a carrier for transmissions from thebasestation, wherein the carrier selected for transmissions from thebasestation has a bandwidth that partially overlaps with the carrierfrequencies on which the detected signals are being transmitted by theother basestations; and setting a maximum downlink power fortransmissions from the basestation, wherein the maximum downlink poweris set based at least in part on a frequency offset between (1) a centrefrequency of the selected carrier and (2) one of the identified carrierfrequencies on which the detected signals are being transmitted by theother basestations.
 2. A method as claimed in claim 1, furthercomprising setting a maximum uplink power for transmissions from userequipments attached to the basestation, wherein the maximum uplink poweris set based on a frequency offset between a centre frequency of theselected carrier and identified carrier frequencies on which thedetected signals are being transmitted by the other basestations.
 3. Amethod as claimed in claim 1, comprising selecting the carrier fortransmissions from the basestation, based on the number of macrolayerbasestations that it is able to detect, the carriers they aretransmitting on, and the strength of the detected macrolayer carriers.4. A method of operation of a basestation in a cellular communicationsnetwork, the method comprising, in the basestation: detecting signalstransmitted by other basestations of the cellular communicationsnetwork; identifying carrier frequencies on which the detected signalsare being transmitted by the other basestations; selecting a carrier fortransmissions from the basestation, wherein the carrier selected fortransmissions from the basestation has a bandwidth that partiallyoverlaps with the carrier frequencies on which the detected signals arebeing transmitted by the other basestations; and setting a maximumuplink power for transmissions from user equipments attached to thebasestation, wherein the maximum uplink power is set based at least inpart on a frequency offset between (1) a centre frequency of theselected carrier and (2) one of the identified carrier frequencies onwhich the detected signals are being transmitted by the otherbasestations.
 5. A method as claimed in claim 4, comprising selectingthe carrier for transmissions from the basestation, based on the numberof macrolayer basestations that it is able to detect, the carriers theyare transmitting on, and the strength of the detected macrolayercarriers.
 6. A method of operation of a femtocell basestation in amobile communications network, wherein the mobile communications networkcomprises a plurality of macro layer basestations, each operating in arespective carrier frequency band, the method comprising: detecting acarrier in use by at least one of the macro layer basestations, whereinthe detected carrier has a bandwidth; selecting a carrier having saidbandwidth for use in the femtocell basestation, based on the detectedcarrier in use by the at least one macro layer basestation; and settinga downlink power for transmission from the femtocell basestation,wherein a maximum downlink power is set based at least in part on afrequency offset between 1) a center frequency of the selected carrierand 2) a center frequency of the detected carrier in use by the at leastone macro layer basestation, characterized in that: if it is onlypossible to detect one or more macro layer basestation transmitting on afirst carrier, the selected carrier is a third carrier; if it is onlypossible to detect one or more macro layer basestation transmitting on asecond carrier, the selected carrier is a fourth carrier; if it ispossible to detect macro layer basestations transmitting on the firstcarrier and the second carrier, the selected carrier is a fifth carrier;and if it is possible to detect two adjacent carriers in use by two ofthe plurality of macro layer basestations, wherein the detected carrierseach have said bandwidth and are separated by a guard band, the selectedcarrier straddles the guard band and partially overlaps with both of thedetected carriers.
 7. The method of claim 6, wherein, in a mobilecommunications network in which a channel raster is 200 kHz, and anominal carrier width is 5 MHz, a center frequency of the selectedcarrier is spaced apart from a center frequency of the detected carrierin used by the at least one macro layer basestation by a frequencyoffset in the range of 200 kHz to 4.8 MHz.
 8. A method as claimed inclaim 6, further comprising setting a maximum uplink power fortransmissions from user equipments attached to the basestation, whereinthe maximum uplink power is set based at least in part on a frequencyoffset between (1) a center frequency of the selected carrier and (2)one of a plurality of identified carrier frequencies on which thedetected signals are being transmitted by the other basestations.
 9. Abasestation, for use in a cellular mobile communications network,wherein the cellular mobile communications network comprises a pluralityof other basestations, each transmitting signals on respective carrierfrequencies, and wherein the basestation is adapted to operate inaccordance with the method according to any preceding claim.